The present invention relates to a weight measuring device wherein a load cell is used as a load sensor.
In an electronic scale or an electronic balance using a load cell as a load sensor, as shown in FIG. 5, one end portion of a load cell 51 is fixed to a base 52 and the other end portion thereof is provided with a pan stop 53 as disclosed in, for example, Japanese Patent Publication (Kokai) No. 2000-171289.
The load cell 51 has a strain member 51a having, for example, four strain portions E, and strain gauges S1 to S4 are attached to the strain portions E. As shown in FIG. 6, a Wheatstone bridge is formed of the respective strain gauges S1 to S4. An output value of the Wheatstone bridge is converted into a weight value by using a sensitivity coefficient and is displayed as a calculated value.
The sensitivity coefficient is calibrated and updated by using a weight with a known weight. More specifically, a weight having a known weight is loaded on the pan stop 53 of the load cell 51, and the sensitivity coefficient is updated so that a calculated value corresponds to the known weight. Also, there has been known a weight measuring device equipped with an adding/removing weight mechanism having a built-in weight for calibration, and the built-in weight is placed on a specific position of the load cell 51 as disclosed in, for example, Japanese Patent Publication (Kokai) No. 11-108740.
When the calibration is carried out, normally, a weight with a known weight within a weighing range of the electronic scale or electronic balance is used. Also, the sensitivity calibration is carried out by using a weight below the weighing range. Further, a load cell type weight measuring device has been proposed, wherein a lever mechanism is provided to the load cell 51 so that a weight below the weighing range is loaded, and the sensitivity calibration is carried out in a state that a weight substantially in the vicinity of the weighing range is loaded to the load cell 51 (as disclosed in Japanese Patent Publication (Kokai) No. 11-108740).
In a case that a load cell type weight measuring device has a large weighing range, when a weight in the vicinity of the weighing range is used for the sensitivity calibration, it is necessary to load and unload the weight manually, thereby causing physical burden. Also, when the adding/removing weight mechanism having the built-in weight is used, the weight measuring device tends to be large.
On the other hand, if the sensitivity calibration is carried out by using a weight lighter than a weighing range, it is difficult to calibrate with high accuracy due to a linearity of an output of a load cell (output of a Wheatstone bridge) relative to a load, so that the weight measuring device can not be applied to a load cell with high resolution.
Further, when a weight lighter than a weighing range is loaded to a load cell through a lever mechanism, a structure tends to be complicated and expensive, and it is difficult to carry out the calibration with high precision.
In view of the problems described above, the present invention has been made, and an object of the invention is to provide a load cell type weight measuring device with a simple structure, wherein a sensitivity calibration can be carried out with high precision even when a weight lighter than a weighing range is used.
Another object of the invention is to provide a dual range load cell type weight measuring device with a simple structure.
Further objects and advantages of the invention will be apparent from the following description of the invention.